Hydraulic arrangements with implemented float positions enabling free movement of a hydraulic cylinder are known in current technology. Both connection sides of the hydraulic cylinder are connected not only to each other but also at low or zero pressure to a reservoir or hydraulic container. Such types of hydraulic arrangement are utilized in construction and/or loading vehicles, such as telescopic loading or front loading vehicles, on which a boom or lever can be raised or lowered by means of a lifting cylinder. The function of the float position is utilized for example to enable a tool mounted on the vehicle boom or lever to follow the ground contours with precision independently from vehicle position and location. The tool is thus pressed on the ground by the force of gravity alone.
Designing hydraulic arrangements with float positions incurs high cost particularly if a load holding valve is provided for safety purposes to prevent or significantly decelerate any unforeseen lowering of the boom or lever caused by the occurrence of a leak in the connection between cylinder and controller. Opening or circumventing the load holding valve generally requires a control pressure to open the valve. In the float position the hydraulic cylinder is at zero pressure and no control pressure is available without some additional device. In order to exert this control pressure such types of hydraulic arrangements have to be fitted with additional on/off valves and/or hydraulic pipes. These additional fittings serve as needed to make or break the connection between the rod side of the hydraulic cylinder and the hydraulic reservoir.
DE 10006908A1 describes a hydraulic piston cylinder assembly for agricultural machinery with a load holding valve enabling a working position to be attained in which a constant pressure can be set at the piston base side of the cylinder chamber. Thereby a boom or a tool mounted on it can rest on the ground with a pre-selected contact pressure. To attain this working position the pressure chambers of the piston cylinder assembly are connected to each other and the pressure between the two pressure chambers is equilibrated by means of a pressure regulator. If the pressure falls below a pre-selected level, the regulator closes. A float position here possible only if the pre-selected level is set to zero so that no pressure regulation occurs. This has the disadvantageous effect that when switched off under load the boom or tool descends uncontrollably.
German patent application DE 10307346 describes a valve arrangement which includes a load holding valve and enables a float position for a hydraulic cylinder. To permit lowering the hydraulic cylinder under load, a special on/off valve is provided in an additional hydraulic pipe connected to a hydraulic reservoir. Not until this on/off valve is closed can the necessary control pressure be generated to open the load holding valve. Moreover, in such a type of hydraulic arrangement, undesirable switch settings or incorrect switch activation may conceivably arise to increase the complexity of valve relationships or to lead to operator error.